Method of detecting a substrate in a carrier head

ABSTRACT

A carrier head has a base, a flexible membrane, and a valve in the carrier head that forms part of a substrate detection system. The valve includes a valve stem that contacts an upper surface of the flexible membrane so that if a substrate is attached to the lower surface of the flexible membrane when the first chamber is evacuated, the valve is actuated to generate a signal to the substrate detection system.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation application of and claimspriority to U.S. application Ser. No. 09/441,928, filed on Nov. 17,1999, the entire disclosure of which is incorporated herein byreference.

BACKGROUND

[0002] The present invention relates generally to chemical mechanicalpolishing of substrates, and more particularly to the detection of asubstrate in a carrier head.

[0003] Integrated circuits are typically formed on substrates,particularly silicon wafers, by the sequential deposition of conductive,semiconductive or insulative layers. After each layer is deposited, thelayer is etched to create circuitry features. As a series of layers aresequentially deposited and etched, the outer or uppermost surface of thesubstrate, i.e., the exposed surface of the substrate, becomesincreasingly non-planar.

[0004] Chemical mechanical polishing (CMP) is one accepted method ofplanarizing a substrate surface. This planarization method typicallyrequires that the substrate be mounted to a carrier or polishing head.The exposed surface of the substrate is then placed against a rotatingpolishing pad or moving polishing belt. The polishing pad may be a“standard” pad with a durable roughened surface, or a fixed-abrasive padwith abrasive particles embedded in a binder. The carrier provides acontrollable load on the substrate to press it against the polishingpad. In addition, the carrier may rotate to affect the relative velocitydistribution over the surface of the substrate. A polishing slurry,including at least one chemically-reactive agent, and an abrasive if astandard pad is being used, may be distributed over the polishing pad.

[0005] Typically, the carrier head is used to remove the substrate fromthe polishing pad after the polishing process has been completed. Thesubstrate is vacuum-chucked to the underside of the carrier head. Whenthe carrier head is retracted, the substrate is lifted off the polishingpad.

[0006] One problem that has been encountered in CMP is that thesubstrate may not be lifted by the carrier head. For example, if thesurface tension binding the substrate to the polishing pad is greaterthan the force binding the substrate to the carrier head, then thesubstrate will remain on the polishing pad when the carrier headretracts. Also, if a defective substrate fractures during polishing,then the carrier head may be unable to remove the fractured substratefrom the polishing pad.

[0007] A related problem is that the attachment of the substrate to thecarrier head may fail, and the substrate may detach from the carrierhead. This may occur if, for example, the substrate was attached to thecarrier head by surface tension alone, rather than in combination withvacuum-chucking.

[0008] As such, an operator may not know that the carrier head no longercarries the substrate. The CMP apparatus will continue to operate eventhough the substrate is no longer present in the carrier head. This maydecrease throughput. In addition, a loose substrate, i.e., one notattached to a carrier head, may be knocked about by the movingcomponents of the CMP apparatus, potentially damaging the substrate orthe polishing pad, or leaving debris which may damage other substrates.

[0009] Another problem encountered in CMP is the difficulty ofdetermining whether the substrate is present in the carrier head.Because the substrate is located beneath the carrier head, it isdifficult to determine by visual inspection whether the substrate ispresent in and properly attached to the carrier head. In addition,optical detection techniques are impeded by the presence of slurry.

[0010] A carrier head may include a rigid base having a bottom surfacewhich serves as a substrate receiving surface. Multiple channels extendthrough the base to the substrate receiving surface. A pump or vacuumsource can apply a vacuum to the channels. When air is pumped out of thechannels, the substrate will be vacuum-chucked to the bottom surface ofthe base. A pressure sensor may be connected to a pressure line betweenthe vacuum source and the channels in the carrier head. If the substratewas not successfully vacuum-chucked to the carrier head, then thechannels will be open and air or other fluid will leak into thechannels. On the other hand, if the substrate was successfullyvacuum-chucked to the carrier head, then the channels will be sealed andair will not leak into the channels. Consequently, the pressure sensorwill measure a higher vacuum or lower pressure when the substrate issuccessfully vacuum-chucked to the underside of the carrier head ascompared to when the substrate is not attached to the carrier head.

[0011] Unfortunately, there are several problems with this method ofdetecting the presence of a substrate in the carrier head. Corrosiveslurry may be suctioned into the channels and contaminate the carrierhead. In addition, the threshold pressure for determining whether thesubstrate has been lifted from the polishing pad must be determinedexperimentally.

[0012] Accordingly, it would be useful to provide a CMP system capableof reliably sensing the presence of a substrate in a carrier head. Itwould also be useful if such a system could operate without exposing theinterior of the carrier head to contamination by a slurry.

SUMMARY

[0013] In one aspect, the invention is directed to a carrier head thathas a base, a flexible member that defines a first chamber and has alower face that provides a substrate receiving surface, and a valve inthe carrier head that forms part of a substrate detection system. Thevalve includes a valve stem that contacts an upper surface of theflexible membrane so that if a substrate is attached to the lowersurface of the flexible membrane when the first chamber is evacuated,the valve is actuated to generate a signal to the substrate detectionsystem.

[0014] Implementations of the invention may include the followingfeatures. The valve may be positioned in a passage that fluidly couplesthe first chamber to a second chamber. The valve may be biased in anopen or closed position, and actuation of the valve may close or openthe valve. The valve stem may extend through an aperture in a supportstructure, and may project slightly beyond a lower surface of thesupport structure. The support structure may be movable relative to thebase. The valve may be biased by a spring, and the spring constant ofthe spring may be selected so that the force from the spring issufficient to counteract a force from a flexible membrane when thesubstrate is not attached, but is insufficient to counteract a forcefrom a flexible membrane when the substrate is attached. The valve stemmay contacts the upper surface of the flexible membrane if the firstchamber is evacuated. The flexible membrane may wrap around a lowerportion of the valve if the substrate is not present.

[0015] In another implementation, the carrier head has a base, aflexible member that defines a first chamber and has a lower face thatprovides a substrate receiving surface, and a valve in the carrier headthat forms part of a substrate detection system. The valve includes avalve stem that projects past a support surface, so that if the firstchamber is evacuated and a substrate is attached to the lower surface ofthe flexible membrane, the substrate abuts the support surface andactuates the valve.

[0016] In another implementation, the carrier head has a base, aflexible member that defines a first chamber and has a lower face thatprovides a substrate receiving surface, and a plurality of valves in thecarrier head that form part of a wafer detection system. If a substrateis attached to the flexible membrane when the first chamber isevacuated, either of the valves may be actuated to generate a signal tothe wafer detection system.

[0017] In another implementation, the carrier head has a base, aflexible member that defines a first chamber and has a lower face thatprovides a substrate receiving surface, and a plurality of valves in thecarrier head that form part of a wafer detection system. If a substrateis attached to the flexible membrane when the first chamber isevacuated, both of the valves must be actuated to generate a signal tothe wafer detection system.

[0018] In another implementation, the carrier head has a base, aflexible member that defines a first chamber and has a lower face thatprovides a substrate receiving surface, a second chamber, a passagethrough the base between the first and second chambers, a first valvethat is biased open and actuates to close the passage if the firstchamber is evacuated a substrate is attached to the flexible membranewhen the first chamber, and a second valve connected in series with thefirst valve, the second valve biased closed and actuatable to open thepassage if the second chamber is evacuated.

[0019] Advantages of the invention include the following. The CMPapparatus includes a sensor to detect whether the substrate is properlyattached to the carrier head. The sensor is less prone to false alarms.

[0020] Other advantages and features of the invention become apparentfrom the following description, including the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is an exploded perspective view of a chemical mechanicalpolishing apparatus.

[0022]FIG. 2 is partially a schematic cross-sectional view of a carrierhead with a flexible membrane and a chamber, and partially a schematicdiagram of a pneumatic control system for the carrier head.

[0023]FIG. 3A is an expanded view of the valve from the carrier head ofFIG. 2.

[0024]FIG. 3B is a view of the carrier head of FIG. 3A with an attachedsubstrate.

[0025]FIG. 4 is a graph showing pressure as a function of time in a CMPapparatus using the carrier head of FIG. 2.

[0026]FIG. 5 is a schematic cross-sectional view of a carrier head thatincludes multiple valves connected in parallel.

[0027]FIG. 6 is a schematic cross-sectional view of a carrier head thatincludes multiple valves connected in series.

[0028]FIG. 7 is a schematic cross-sectional view of a carrier head inwhich valves are separated by a diaphragm.

[0029]FIG. 8 is a schematic cross-sectional view of a carrier head inwhich valves are biased in opposite directions.

[0030] Like reference numbers are intended in the various drawings toindicate like elements, although some elements in differentimplementations may have different structures, operations or functions.

DETAILED DESCRIPTION

[0031] Referring to FIG. 1, one or more substrates 10 will be polishedby a chemical mechanical polishing (CMP) apparatus 20. A completedescription of a CMP apparatus can be found in pending U.S. Pat. No.5,738,574, the entire disclosure of which is hereby incorporated byreference. The CMP apparatus 20 includes a series of polishing stations25 and a transfer station 27.

[0032] Each polishing station 25 includes a rotatable platen 30 on whichis placed a polishing pad 32. Each polishing station may further includean associated pad conditioner apparatus 34 to periodically reconditionthe polishing pad surface. Each polishing station can also include acombined slurry/rinse arm 36 to supply a slurry 38 containing an activeagent (e.g., deionized water for oxide polishing), abrasive particles(e.g., silicon dioxide for oxide polishing) and a chemically-reactivecatalyzer (e.g., potassium hydroxide for oxide polishing) to the surfaceof polishing pad 32.

[0033] The CMP apparatus 20 also includes a rotatable multi-headcarousel 40 that supports four carrier heads 100. Three of the carrierheads receive and hold substrates and polish them by pressing themagainst the polishing pad 32 on platen 30 of polishing stations 25. Oneof the carrier heads receives a substrate from and delivers thesubstrate to transfer station 27. The carousel can rotate to orbit thecarrier heads, and the substrates attached thereto, between thepolishing stations and the transfer station. Each carrier can beindependently rotated about its own axis, and independently laterallyoscillated by a drive shaft 42.

[0034] Generally, carrier head 100 holds the substrate against thepolishing pad and evenly distributes a force across the back surface ofthe substrate. The carrier head also transfers torque from the driveshaft to the substrate and ensures that the substrate does not slip frombeneath the carrier head during polishing.

[0035] Referring to FIG. 2, carrier head 100 includes a housing hub 102,a base 104, a loading chamber 108, a retaining ring 110, and a substratebacking assembly 112. Descriptions of similar carrier heads may be foundin U.S. Pat. No. 5,957,751, and in pending U.S. application Ser. No.09/169,500, filed Oct. 9, 1998, each of which is incorporated herein byreference in its entirety.

[0036] The housing hub 102 is connected to drive shaft 42 to rotatetherewith about an axis of rotation which is substantially perpendicularto the surface of the polishing pad. Three passages 130, 132 and 134 areformed through housing hub 104 for pneumatic control of the carrierhead.

[0037] Base 104 includes a gimbal mechanism 106 and an outer clamp ring144. The vertical position of base 104 relative to housing hub 102 iscontrolled by loading chamber 108. Chamber 108 also controls thedownward pressure on base 104 and retaining ring 110. Loading chamber108 is sealed by a diaphragm 140 that is clamped to housing hub 102 byan inner clamp ring 142 and clamped to base 104 between outer clamp ring144 and flexure ring 152. Outer clamp ring 144 includes an inwardlyprojecting flange 146 which extends over a lip of housing hub 102 toprevent over-extension of the carrier head and to prevent slurry fromcontaminating diaphragm 140.

[0038] A first pump or pressure source 52 a may be connected to loadingchamber 108 via passage 130 in housing hub 102. If pump 52 a pumps fluidinto loading chamber 108, the volume of the chamber will increase andbase 104 will be pushed downwardly. On the other hand, if pump 52 apumps fluid out of loading chamber 108, the volume of chamber 108 willdecrease and base 104 will be pulled upwardly.

[0039] Gimbal mechanism 106 permits base 104 to move with respect tohousing hub 102 so that the retaining ring may remain substantiallyparallel with the surface of the polishing pad. Gimbal mechanism 106includes a gimbal rod 150 and a flexure ring 152. Gimbal rod 150 mayslide vertically in passage 132 in housing 102 so that base 104 can movevertically with respect to housing 102. However, gimbal rod 150 preventsany lateral motion of base 104 with respect to housing 102. A firstpassage 154 can be formed through gimbal rod 150, and a second passage156 can be formed through gimbal rod 150, flexure ring 152 and outerclamp ring 144 for pneumatic control of the carrier head.

[0040] Retaining ring 110 may be secured at the outer edge of base 104.Retaining ring 110 can have a flat bottom surface 126, or the bottomsurface can include channels to permit slurry flow. When fluid is pumpedinto chamber 108 and base 104 is pushed downwardly, retaining ring 110is also pushed downwardly to apply a load to polishing pad 32. An innersurface of 124 retaining ring 110 restrains the substrate from lateralmotion.

[0041] A membrane 162 may be clamped to a lower surface of base 104 by aclamp ring 164 to form an annular bladder 160. A passage 166 extendsthrough clamp ring 164 and is aligned with passage 156 in base 104. Asecond pump or pressure source 52 b can connected to bladder 160 viapassage 134 in housing hub 102, passage 156 in base 104, and passage 166in clamp ring 164. If pump 52 b forces a fluid into bladder 160, thenbladder 160 will expand downwardly. On the other hand, if pump 52 bevacuates fluid, then bladder 160 will contract. As discussed below,bladder 160 may be used to apply a downward pressure to supportstructure 114 and flexible membrane 118.

[0042] The substrate backing assembly 112 includes a flexible membrane118, a support ring 116, a support structure 114, and a spacer ring 128.Each of these elements will be explained in greater detail below.

[0043] Flexible membrane 118 is a generally circular sheet formed of aflexible and elastic material with a central portion 170 and aperipheral portion 172 that extends between spacer ring 128 and supportplate 114. The central portion 170 of flexible membrane 118 extendsbelow support structure 114 to provide a mounting surface for thesubstrate. An inner edge of the peripheral portion 172 is folded backover the perimeter of the central portion 170 to form an expandable lip174, as discussed in pending U.S. application Ser. No. 09/296,935, filedApr. 22, 1999, the entirety of which is incorporated herein byreference. An outer edge of membrane 118 is clamped between retainingring 110 and outer clamp ring 144 to define a pressurizable chamber 120.

[0044] A third pump or pressure source 52 c can be connected to chamber120 via passage 154 in gimbal rod 150. If pump 52 c forces a fluid intochamber 120, then the volume of the chamber will increase and flexiblemembrane 118 will be forced downwardly. On the other hand, if pump 52 cevacuates air from chamber 120, then the volume of the chamber willdecrease and the membrane will be drawn upwardly.

[0045] Spacer ring 128 is an annular body positioned between supportstructure 114 and retaining ring 110 to maintain the proper shape offlexible membrane 118. Spacer ring 128 can rest on the lip portion offlexible membrane 118.

[0046] Support ring 116 is an annular piece with a C-shapedcross-section that rests inside chamber 120 on flexible membrane 118.The central portion 170 of flexible membrane 118 can include an inwardlyextending flap 176 that engages support ring 116 to maintain the propershape of flexible membrane 118.

[0047] Support structure 114 also rests inside chamber 120 on flexiblemembrane 118. The support structure 114 includes a disk-shaped plateportion 180 with a plurality of unillustrated apertures, an outwardlyextending flange portion 182 that extends over support ring 116, and adownwardly extending flange portion 184 that extends between supportring 116 and peripheral portion 172 of flexible membrane to rest on thecentral portion 170 of the flexible membrane.

[0048] The CMP apparatus of the present invention is capable ofdetecting whether a substrate is properly attached to carrier head 100.If the CMP apparatus detects that the substrate is missing or isimproperly attached to the carrier head, the operator may be alerted andpolishing operations may be automatically halted.

[0049] Three pressure sensors or gauges 56 a, 56 b and 56 c may beconnected to the fluid lines between pumps 52 a, 52 b and 52 c, andchambers 108, 160, and 120, respectively. Controllable valves 58 a, 58 band 58 c may be connected across the fluid lines between pressure gauges56 a, 56 b and 56 c, and pumps 52 a, 52 b and 52 c, respectively. Pumps52 a-52 c, pressure gauges 56 a-56 c and valves 58 a-58 c may beappropriately connected to a general-purpose digital computer 60.Computer 60 may operate pumps 52 a-52 c, as described above, topneumatically power carrier head 100 and to vacuum-chuck a substrate tothe bottom of the carrier head. In addition, computer 60 may operatevalves 58 a-58 c and monitor pressure gauges 56 a-56 c, as described inmore detail below, to sense the presence of the substrate in the carrierhead.

[0050] Referring to FIGS. 3A and 3B, the carrier head 100 includes amechanically actuated valve 200 to provide the carrier head with a waferdetection capability. In one implementation, passage 156 is connected toa chamber 220 in flexure ring 152, and valve 200 is positioned near thecenter of the carrier and extends between chamber 220 and chamber 120.In this implementation, valve 200 includes a valve stem 202, an annularflange 204 that extends radially outwardly from the valve stem 202, anO-ring 206, and a spring 214. Valve stem 202 extends through an aperture208 in flexure ring 152 between valve chamber 220 and lower chamber 120,with valve flange 204 positioned in valve chamber 220. The portion ofvalve stem 202 that extends into lower chamber 120 passes through anaperture 210 in support structure 114. When lower chamber 120 isevacuated and support structure 114 is retracted against base 104, valvestem 202 can extend slightly below a bottom surface 186 of supportstructure 114. Channels 212 may be formed in flexure ring 152surrounding aperture 208 and valve stem 202 to connect chamber 120 tovalve chamber 220. However, O-ring 206 is positioned around valve stem202 in valve chamber 220 between annular flange 204 and flexure ring152. In addition, spring 214 is positioned between annular flange 204and a ceiling 222 of valve chamber 220. Spring 214 biases the valve 200into a closed position. (as shown in FIG. 3A). More specifically, O-ring206 is compressed between annular flange 204 and flexure ring 152 toseal channels 212 from valve chamber 220, thereby isolating valvechamber 220 from lower chamber 120. However, if valve stem 202 is forcedupwardly (as shown in FIG. 3B), then O-ring 206 will no longer becompressed and fluid may leak through a gap 218 around the O-ring. Assuch, valve 200 will be open and valve chamber 220 and lower chamber 120will be in fluid communication via channels 212.

[0051] A CMP apparatus including carrier head 100 senses whether thesubstrate has been successfully vacuum-chucked to the carrier head asfollows. The substrate is positioned against the flexible membrane 118.Pump 52 b inflates bladder 160 to a predetermined pressure, and thenvalve 58 b is closed to isolate bladder 160 from pump 52 b. A firstmeasurement of the pressure in bladder 160 is made by means of pressuregauge 56 b. Then pump 52 c evacuates lower chamber 120 to create alow-pressure pocket between the flexible membrane and the substrate inorder to vacuum chuck the substrate to the carrier head. Then a secondmeasurement of the pressure in bladder 160 is made by means of pressuregauge 56 b. The first and second pressure measurements may be comparedto determine whether the substrate was successfully vacuum-chucked tothe carrier head.

[0052] Carrier head 100 is configured so that valve 200 will actuate ifthe substrate is present, and will not actuate if the substrate isabsent. As shown in FIG. 3A, if the substrate is not present, then whenchamber 120 is evacuated, flexible membrane 118 will move upwardly andcontact the valve stem. However, since flexible membrane 118 is flexibleand is partly supported against support structure 114 when chamber 120is evacuuate, the flexible membrane will tend to wrap around the valvestem, and the upward tension force on valve stem 202 from flexiblemembrane 118 will be insufficient to overcome the downward spring forcefrom spring 204, and the valve 200 will remain closed. On the otherhand, as shown in FIG. 3B, if the substrate is vacuum-chucked to theflexible membrane, the relatively rigid substrate will press on valvestem 202. In this case, the upward tension force from flexible membrane118 and substrate 10 will overcome the downward spring force from spring204, and the valve 200 will open, thereby fluidly connecting lowerchamber 120 to valve chamber 220. This permits fluid to be drawn out ofbladder 160 through valve chamber 220 and lower chamber 120, andevacuated by pump 52 c.

[0053] It should be noted that spring 204 is selected to provide adownward force that is sufficient to counteract the upward force appliedby the membrane alone, but insufficient to counteract the upward forceapplied when a substrate is attached to the membrane. In general, thelarger the aperture 210 in support structure 114, the stiffer themembrane 118, and the farther the valve stem 202 extends past lowersurface 176, the more force flexible membrane 118 will apply to thevalve stem 202, and the larger the spring constant of spring 204 willneed to be. However, a lower spring constant results in less stress onthe substrate as the valve is actuated.

[0054] Referring to FIG. 4, bladder 160 may initially be at a pressureP₁. The first pressure measurement is made at time T₁ before pump 52 cbegins to evacuate lower chamber 120. When chamber 120 is evacuated attime T₂, flexible membrane 118 is drawn upwardly. If the substrate ispresent, valve 200 remains closed, and the pressure in bladder 160 willremain constant at pressure P₁, or even rise to a pressure P₂ if supportstructure 114 applies an upward force to compress the bladder 160. Thus,the pressure in bladder 160 measured by gauge 56 b will remain at orabove pressure P₁. On the other hand, if the substrate is present, thenvalve 200 is opened and fluid is evacuated from volume 160 so that thepressure measured by gauge 56 b falls to pressure P₃. Therefore, if thesecond measured pressure is less than the first measured pressure, thesubstrate is attached to the carrier head. However, if the secondmeasured pressure is equal to or larger than the first measuredpressure, the substrate is not attached to the carrier head.

[0055] Computer 60 may be programmed to store the two pressuremeasurements, compare the pressure measurements, and thereby determinewhether the substrate was successfully vacuum-chucked to the carrierhead. This can provide an extremely reliable substrate detector that isnot subject to “false” signals, e.g., indications that the substrate isabsent when it is, in fact, present. In addition, the sensor iscontained within the carrier head behind the flexible membrane, so thatthe sensor does not provide an opportunity for slurry to contaminate theinterior of the carrier head.

[0056] Referring to FIG. 5, in another implementation, carrier head 100a includes two or more valves 300, 310 connected in parallel betweenlower chamber 120 and bladder 160. For example, the first valve canextend between lower chamber 120 and a first chamber 302, whereas thesecond valve can extend between lower chamber 120 and a second chamber312. A passage 320 in flexure ring 154 can connect first chamber 302 tosecond chamber 312. Thus, chamber 120 will be connected to bladder 160if either or both valves 300 is triggered. This implementation increasesthe sensitivity of the carrier head to the presence of the wafer, andprovides redundancy in case one valve becomes stuck. In addition, if thecarrier head includes three or more valves spaced at equal angularintervals around the carrier base, the substrate will not be tilted asit is lifted.

[0057] Referring to FIG. 6, in another implementation, carrier head 100b includes two or more valves 400, 410 connected in series betweenchamber 120 and bladder 160. For example, the first valve can extendbetween lower chamber 120 and a first chamber 402, and a passage 420through flexure ring 152 can connect first chamber 402 to a passage 414that is sealed from a second chamber 412 by the O-rings of second valve410. The second chamber 412 is connected to bladder 160 by passage 156.In short, the input of first valve 400 is connected to chamber 120, theoutput of the first valve 400 is connected to the input of second valve402 by passage 420, and the output of second valve 410 is connected tobladder 160. Thus, chamber 120 will be connected to bladder 160 only ifboth valves 400 and 410 are triggered. This implementation increases thesensitivity of the carrier head to the absence of the substrate and tosituations in which the substrate is not sufficiently firmly secured tothe flexible membrane, e.g., if the substrate is attached to theflexible membrane by surface tension alone, and not by vacuum-chucking,and tilts rather than actuating both sensors. The input passage 414 ofsecond valve 410 can be separated from chamber 120, while allowing thevalve stem of second valve 410 to extend into the chamber 120, byO-rings 416.

[0058] As shown in FIG. 7, a flexible diaphragm 430 can be used insteadof O-rings to separate passage 414 of second valve 410′ from the chamber120. Valve stem 202′ of valve 410′ can rest on diaphragm 430, and abumper 432 can be affixed to the underside of diaphragm 430. Flexiblediaphragm 430 is sufficiently elastic that when bumper 432 is pressedupwardly by flexible membrane 118, bumper 432 can be forced up intoaperture 210′ in support structure 114, thus forcing valve stem 202′upwardly to actuate second valve 410′.

[0059] Referring to FIG. 8, in another implementation, carrier head 100c includes two valves 500 and 510 connected in series between chambers120 and 108. This implementation would be appropriate for the carrierhead discussed in pending U.S. Application Serial No. 60/114,182, filedDec. 30, 1998. In this implementation, the valves 500 and 510 can beformed between flexure ring 152′ and an annular gimbal clamp 158, andmultiple fluid passages through the gimbal rod and the flexure ring arenot required. First valve 500 fluidly connects chamber 120 to a firstvalve chamber 502 via channels 508 in flexure ring 152′ surroundingvalve stem 506, second valve 510 fluidly connects chamber 108 to asecond valve chamber 512 via channels 518 in gimbal clamp 158surrounding valve stem 516, and first valve chamber 502 is connected tosecond valve chamber 512 by an unillustrated passage. The first valve500 is biased open by spring 504, and second valve 510 is biased closedby spring 514. If the lower chamber 120 is evacuated and a substrate isvacuum-chucked to the carrier head, then valve stem 506 of first valve500 will be actuated to press O-ring 506 against gimbal clamp 158 toclose the first valve, and the pressure in chamber 108 will remainconstant. On the other hand, if the lower chamber 120 is evacuated butno substrate is present, then first valve 500 will remain open. Ifloading chamber 108 is also evacuated when chamber 120 is evacuated,e.g., in order to lift the entire substrate backing assembly and theretaining ring away from the polishing pad, then valve stem 516 will bepressed against housing hub 102. This generates a downward force on thevalve stem which can overcome an upward force from spring 514 thatpresses O-ring against gimbal clamp 158, causing the second valve 510 toopen and thus connecting loading chamber 108 to lower chamber 120. Fluidwill then flow out of loading chamber 108 via lower chamber 120. On theother hand, if the loading chamber 108 is pressurized when chamber 120is evacuated, e.g., to control the contact area and pressure on thesubstrate during polishing, then valve 510 will remain closed. In sum,the valve assembly will be actuated to connect loading chamber 108 tolower chamber 120 only if a substrate is not present and chamber 108 isevacuated. The drop in pressure in lower chamber 120 can be detected bypressure gauge 52 c to indicate that the substrate is not present.

[0060] Alternatively, carrier head 100 c could include a single valvethat opens when chamber 120 is evacuated if a substrate is present. Inthis case, the valve that separates chamber 108 from a pump or pressuresource can remain open so that chamber 120 does not entirely evacuate,thus preventing the membrane 118 from pull so far into chamber 120 thatthe substrate becomes overstressed and damaged.

[0061] Although in several implementations the valves are described asconnecting lower chamber 120 to bladder 160, the valve could be used toconnect any two chambers in the carrier head, or the valve can connect achamber in the carrier head to the ambient atmosphere. Moreover, thevalve can be biased opened or closed, so that the presence of thesubstrate can either close or open the valve, respectively, when thevalve is actuated. The valve can be positioned in parts of the carrierhead other than the flexure ring. For example, the valve can be offsetfrom the center of the carrier and attached to a base ring with thevalve chamber formed between the flexure ring and the base ring. Inaddition, the passages formed through the carrier head to provide thefluid connections are exemplary. For example, fluid communication can beprovided by a flexible hose that is coupled to fixtures on the housinghub and base ring, a first passage can connect the fixture on the basering to the valve chamber, and a second passage can connect the valvechamber to the bladder.

[0062] The present invention has been described in terms of a number ofpreferred embodiments. The invention, however, is not limited to theembodiments depicted and described. The scope of the invention isdefined by the appended claims.

What is claimed is:
 1. A method for detecting the presence of asubstrate in a carrier head, comprising: placing a substrate against asubstrate receiving surface of a flexible membrane in the carrier head,the flexible membrane defining a boundary of first chamber; evacuatingthe first chamber, wherein if a substrate is attached to the lowersurface of the flexible membrane when the first chamber is evacuated, afirst movable structure that forms part of a substrate detection systemis actuated; determining whether the substrate is chucked to thesubstrate receiving surface depending on whether the first movablestructure was actuated.
 2. The method of claim 1, wherein determiningwhether the substrate is chucked includes measuring a first pressure ofa volume in the carrier head before the evacuating step and measuring asecond pressure of the volume in the carrier head after the evacuatingstep, and comparing the first and second pressures.
 3. The method ofclaim 2, wherein actuating the first movable structure causes the secondpressure to be a different pressure than if first movable structure werenot actuated.
 4. The method of claim 2, wherein the first movablestructure contacts an inner surface of the flexible membrane.
 5. Themethod of claim 2, wherein the first movable structure comprises aportion of a valve.
 6. The method of claim 5, wherein the valveregulates fluid flow through a passage in the carrier head.
 7. Themethod of claim 6, wherein the passage that fluidly couples the firstchamber to a second chamber.
 8. The method of claim 5, wherein the valveis biased in a closed position, and actuation of the valve opens thevalve.
 9. The method of claim 5, wherein the valve is biased in an openposition, and actuation of the valve closes the valve.
 10. The method ofclaim 5, wherein the valve is biased by a spring, and the springconstant of the spring is selected so that the force from the spring issufficient to counteract a force from a flexible membrane when thesubstrate is not attached, but is insufficient to counteract a forcefrom a flexible membrane when the substrate is attached.
 11. The methodof claim 5, wherein the portion of the valve extends through an aperturein a support structure.
 12. The method of claim 11, wherein the portionof the valve extends slightly beyond a lower surface of the supportstructure.
 13. The method of claim 11, wherein the support structure ismovable relative to the base.
 14. The method of claim 5, wherein theportion of the valve contacts the upper surface of the flexible membraneif the first chamber is evacuated.
 15. The method of claim 5, whereinthe flexible membrane wraps around a lower portion of the valve if thesubstrate is not present.
 16. A method for detecting the presence of asubstrate in a carrier head, comprising: placing a substrate against asubstrate receiving surface of a flexible membrane in the carrier head,the flexible membrane defining a boundary of first chamber; evacuatingthe first chamber, wherein if a substrate is attached to the lowersurface of the flexible membrane when the first chamber is evacuated, atleast one of a first movable structure and a second movable structurethat form part of a substrate detection system is actuated; determiningwhether the substrate is chucked to the substrate receiving surfacedepending on whether the first movable structure was actuated.
 17. Themethod of claim 16, whether the determining step indicates that thesubstrate is chucked if either the first movable structure or the secondmovable structure are actuated.
 18. The method of claim 16, whether thedetermining step indicates that the substrate is chucked if both thefirst movable structure and the second movable structure are actuated.